Modern analogue addressable fire alarm systems use many loop powered alarm sounders controlled by microcontrollers, to alert people in protected areas to the presence of a fire alarm condition. Many alarm sounders use piezo-electric transducers (a piezo) to reduce the current consumption of the sounders in the alarm condition. Typically these analogue addressable systems can continuously monitor all outstation types on each addressable loop for faults, to ensure the system can be relied on to detect fires and alert people. In the case of alarm sounders, the actual sounder output can normally only be switched on and verified during regular tests with the system in the alarm state.
While it would be an enormous benefit to continuously verify that the alarm sounder can actually provide its correct output, background monitoring has always proved difficult to successfully implement especially with sounders using a piezo element. In known systems, complex monitoring waveforms need to be generated, so that background monitoring is normally only available on speech variants. However, as a relatively large acoustic output during the background monitoring has always proved to be unavoidable, its use in bedrooms for example, is clearly unacceptable.
One way to guarantee reliable fault detection of the sounder would be to require that the monitoring frequency be fixed at a relatively low in-band frequency. This configuration would produce a monitoring current high enough to provide reliable discrimination. This however would prevent the monitoring from being silent, and it would limit its general use.